Conventionally, a handover of a mobile terminal is initiated if the mobile terminal receives a signal power from a target cell, which is a hysteresis (H) dB higher or lower than a signal power received from a serving cell for more than time-to-trigger (TTT) seconds.
In a telecom system having a high traffic load in, for example, a Manhattan-like environment, users moving with a relatively high speed, e.g. 50 km/h, will experience considerable handover failure in their mobile terminals when they are turning around street corners. However, for low speed users moving with a speed of e.g. 3 km/h, the handover performance is generally good. It has been shown that most handover failures in such a scenario are caused by unsuccessful reception of handover commands. The main reason for these failures is a delayed initiation of the handover, due to e.g. the so-called “corner effect”, which causes the signal strength from the serving cell to drop as much as 20 to 30 dB within 1 to 2 seconds, and the downlink interference to increase rapidly as the target cell suddenly comes in LOS (Line Of Sight) from the mobile terminal.
In order to reduce such handover failures, a faster handover triggering can be enabled by changing to more “aggressive” mobility parameters, i.e. a shorter TTT and/or smaller H. However, the use of such aggressive parameters also creates many unwanted so-called ping-pong handovers, i.e. when mobile terminals are unnecessarily handed over to and from neighbour cells.
Therefore, such aggressive parameters should only be used in adequate situations, such as for example when a mobile terminal has turned or initiated a turn around a corner in a Manhattan-like scenario. However, no satisfactory method to decide when such an adequate situation arises is known today.
A velocity based corner detecting method is proposed in “Velocity Adaptive Handoff Algorithms for Microcellular Systems”, by Mark D. Austin, Gordon L. Stüber, Vehicular Technology, IEEE Transactions on, in Volume 43, Issue 3, August 1994 Page(s):549-561
In the methods proposed by Austin and Stüber, velocity estimates are used to detect if a mobile terminal has turned a corner or not. Upon detection of the fact that the UE has turned a corner, fast handover is triggered by using a lower hysteresis (H) for L meters or seconds, after which the original hysteresis is reset. This method may, in some situations, counteract the corner effect, but, all velocity based methods will have a significant bias when plane waves arrive from the perpendicular direction with high directivity, as for example when downlink signals from a base station in a street canyon perpendicular to the direction of movement of a mobile terminal, reach the mobile terminal in an intersection in a Manhattan-like environment. In other words, it is difficult to accurately determine the velocity of an object based on directional signals arriving from a direction perpendicular to the objects direction of movement, due to that the object may appear to be in approximately the same position in relation to the emitter of the incoming signals for some time. Therefore, these methods cannot be relied on to make correct estimations in all situations.